Field of the invention
The present invention relates to a semiconductor light emitting device, particularly, to a semiconductor light emitting device having a short wavelength emission.
Description of the related Art
Recently, a light emitting diode (LED) having high emitting efficiency and high luminance is widely researched, for example, for an outdoor display or a traffic signal. However, in a short wavelength region, it is difficult to obtain high efficiency and high luminance. An LED using a new material as an active layer which has a wide band-gap and can be epitaxially grown is researched.
A mixed crystal, InGaAlP is a possible material, because a lattice of InGaAlP can be aligned with a GaAs substrate. The energy gap of InGaAlP is wide and the emission of InGaAlP is a direct transition type. An LED using InGaAlP as an active layer is expected to be an LED for short wavelength emission such as yellow emission or green emission.
The light emitting efficiency of an LED using InGaAlP is not sufficiently high in the short wavelength region. This is because in order to achieve an emission of a short wavelength it is necessary to widen an energy gap. For broadening the energy gap it is necessary to increase the amount of Al in the InGaAlP. However increasing the amount of Al generates a non-radiative center in a semiconductor layer. This is because in growing a semiconductor layer of InGaAlP, Al is chemically reactive and takes oxygen into the layer. This oxygen becomes a non-radiative center.
For example, the emission-efficiency of a yellow LED using InGaAlP (In.sub.0.5 (Ga.sub.0.7 Al.sub.0.3).sub.0.5 P) is about 1% but the efficiency of a green LED using InGaAlP (In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P) of which Al content is larger than that of the yellow LED is only about 0.3%.
In order to improve emission-efficiency, a reflective layer is used, such as a Bragg reflective layer. Light is emitted from the front and back surface of an emission region. The light from the back does not contribute on the efficiency. Therefore, the reflective layer is arranged on the back side of the emission region so as to reflect the light emitted from the back surface. Further a current spreading layer or a current blocking layer is used for avoiding emission from under an upper electrode which causes energy loss. These features are discribed in U.S. Pat. No. 5,153,889. However the improvements resulting from these features are not sufficient.